This invention relates to a multilayer wiring board in which a plurality of wiring layers are laminated on a ceramic or hard resin substrate by using polyimide resin as the interlaminar insulating material and a method of producing same. The multilayer wiring board is suitable for mounting thereon LSI chips.
Multilayer printed wiring boards have been long used as wiring boards for mounting thereon LSI chips. Usually a multilayer printed wiring board is constructed by alternately laminating copper cladded core boards and prepreg sheets, uniting the laminated components by hot pressing to cause the prepreg sheets to function as adhesive, drilling a plurality of through-holes in the united multilayer board and plating the inner surfaces defining the respective through-holes with copper to make interlaminar electrical connections between the core boards.
In the conventional multilayer printed wiring boards the wiring density is restricted by the drilling of the through-holes. It is impossible to extremely reduce the diameter of the through-holes because of forming them by drilling, and hence there is a strict limitation to the number of wiring lines that can be formed in the area between the through-holes. Further, the capacity of each core board for signaling wiring lines becomes lower as the total number of the core boards is increased since the total number of the through-holes must be progressively increased as the number of the core boards increases.
Recently multilayer wiring boards of a different type have been developed to meet the demand for multilayer wiring boards which can be made higher in wiring density and can be used in large computers. A multilayer wiring board of the new type uses polyimide resin as the interlaminar insulating material to support a plurality of wiring layers on a ceramic substrate, and this multi-layer wiring board is produced by alternately repeating a process for forming a polyimide insulating layer and another process for forming a wiring layer. The former process includes applying a polyimide varnish to the substrate or a precedingly formed wiring layer, drying the varnish, forming via holes in the polyimide layer and then curing the polyimide. The latter process includes forming a wiring pattern by photolithography and then making wiring by vacuum evaporation or plating.
However, the above laminating process for producing the polyimide multilayer wiring board takes very long hours since the steps of applying a polyimide varnish, forming via holes and curing the polyimide need to be repeated the same number of times as the total number of the insulating layers. Besides, the repetition of the curing operation is liable to cause the precedingly cured polyimide to deteriorate under repeated thermal stress. Further, it is difficult to raise the yield of acceptable products since the laminating process is a sequential process.
To obviate the disadvantage of the sequential laminating process in respect of the yield of acceptable products there is another method of producing a polyimide multilayer wiring board by using a number of polyimide sheets on each of which a wiring pattern is formed in advance. On a ceramic substrate the polyimide sheets are laminated one after another each time by application of heat and pressure. By this method it is possible to select defectless sheets before the laminating operation, and hence it is possible to raise the yield of acceptable multilayer wiring boards. However, this method also has disadvantages that the laminating process takes long hours because of the need of repeating the heat pressing operation many times and that the polyimide of the bottom layers is liable to deteriorate by repeated thermal stress.